1. Field of the Invention
The invention relates to a force-measuring ring or load cell comprising an annular housing which contains at least one piezoelectric measuring element, and further comprising a pressure transmission element which is attached to said housing, preferably by welding, via inner and outer circular-annular membrane areas. The invention further relates to devices for measuring forces which are acting axially upon a shaft or a stamp.
2. The Prior Art
Force-measuring rings are used to measure forces, pressures, accelerations and moments, and for this purpose are inserted between force-transmitting means and engine parts.
From WO 2004/070334 A1 for instance there is known a measuring sensor with a pre-tensioning device, which essentially consists of an adapter part and an adapter sleeve. The adapter part has an integrated central tensioning sleeve, which extends into a central bore of the adapter sleeve and may be pre-tensioned there by means of a nut. A ring-shaped receiving space of the adapter sleeve carries a force-measuring ring, the measuring elements of the force-measuring ring being disposed in an annular housing and being covered by a membrane, which lies against the adapter part. The membrane is attached to the sensor housing by means of welding and is thus pre-tensioned.
FIG. 1 gives a more detailed description of the known force-measuring ring, which is for instance presented in G. GAUTSCHI, “Piezoelectric Sensors”, Springer 2002. The force-measuring ring 1 has an annular housing 2, which forms a receiving space bounded by an inner flange 8 and an outer flange 9, and contains two piezoelectric measuring elements 5 and a pick-up electrode 6 disposed between those elements. The pick-up electrode 6 connects to a socket element 7 attached to the outer flange 9. The force acting on the force-measuring ring 1 is transmitted to the piezoelectric measuring elements 5 via the pressure transmission element 11. The pressure transmission element 11 has an inner membrane area 12 and an outer membrane area 13, which are welded to the inner flange 8 and the outer flange 9 of the housing 2, respectively. Force-measuring rings of this kind are suitable in conditions when the force to be measured is uniformly applied over the contact surface of the pressure transmission element 11. Such measurement conditions are described for instance in WO 2004/070334 A1 cited above.
There are applications, however, in which no uniform force distribution across the surface of the pressure transmission elements may be assumed. Measuring the axial force on a ball bearing as shown in FIG. 2 would be an example of such an application. In that case the force must be taken off from one of the two ball cages.
FIG. 2 shows in cross-section a rotating shaft 20 with a load in axial direction, the shaft axis being indicated by 20′. The ball bearing 21 sitting on the rotating shaft has an inner cage 22 and an outer cage 23, in which the balls 24 are held. In order to transmit force to the force-measuring ring 1, an annular intermediary part 25 is required to distribute the force uniformly over the force-measuring ring 1. Disadvantages of this arrangement are the great height and the higher elasticity of the measuring assembly.
It is the object of the present invention to improve a force-measuring ring of the kind described above in such a way that it may be used satisfactorily even under space limitations and with asymmetrical force input. In particular, the aim is to develop a robust force-measuring ring for measuring axial forces acting on an axial ball bearing.